1. Field of the Invention
The present invention relates to an atmospheric pressure sensing apparatus, which includes an atmospheric pressure sensor arranged in a case, which receives a control circuit for controlling an internal combustion engine of a vehicle.
2. Description of Related Art
For instance, when a vehicle is traveling along a road climbing a mountain, the atmospheric pressure will change due to a change in an altitude to cause a change in a density (a mass flow rate) of intake air of an internal combustion engine of the vehicle. To counteract this, it has been proposed to correct an air-fuel ratio control parameter, such as a total fuel injection quantity of the internal combustion engine, in accordance with a measured atmospheric pressure value that is measured with an atmospheric pressure sensor to improve an air-fuel ratio control accuracy.
In such a system, which has the atmospheric pressure sensor, the atmospheric pressure sensor is often installed on a circuit board of a control circuit (hereinafter, referred to as an ECU), so that the atmospheric pressure sensor and the ECU are received in a case of the ECU, which is placed in an engine room of vehicle. In this instance, as shown in FIG. 8, when water is applied to the case of the ECU at the time of, for example, washing the vehicle or driving the vehicle on a submerged road, the air, which is present in the case, is rapidly cooled to cause a temporal and rapid drop of the pressure inside the case. Therefore, a measured value of the atmospheric pressure, which is measured with the atmospheric pressure sensor arranged in the case, tends to be temporarily reduced from the actual atmospheric pressure. As a result, at the time of application of water to the case, the atmospheric pressure measurement accuracy of the atmospheric pressure sensor tends to be deteriorated.
To counteract with this, a breathing hole may be formed through a wall of the case of the ECU to communicate between the surrounding atmosphere and an interior of the case and thereby to limit the pressure drop inside the case. Alternatively, the case of the ECU may be placed to a location where there is less chance of getting water.
Furthermore, for example, Japanese Unexamined Patent Publication No. H06-201496 teaches a system, which determines that the atmospheric pressure sensor is abnormal when a rate of change in the measured atmospheric pressure value of the atmospheric pressure sensor becomes equal to or greater than an abnormality determination threshold value.
However, the above measures pose the following disadvantages. That is, in the case of forming the breathing hole in the case of the ECU, a relatively large breathing hole needs to be provided to effectively limit the pressure drop inside the case at the time of application of water to the case, and water could possibly enter the case through such a large breathing hole. To counteract this, a waterproof filter or the like needs to be installed to the breathing hole. In such a case, the air permeability of the breathing hole is disadvantageously reduced by the waterproof filter, and therefore the pressure drop at the time of application of water to the case cannot be sufficiently limited, causing a deterioration of the atmospheric pressure measurement accuracy of the atmospheric pressure sensor.
Furthermore, when the case of the ECU is arranged outside of a passenger compartment (e.g., arranged in the engine room), it is sometimes difficult to provide a space for accommodating the case at a location where water is not applied. Thus, in such a situation, it is not possible to sufficiently limit the pressure drop inside the case at the time of application of water to the case. As a result, the deterioration of the atmospheric pressure measurement of the atmospheric pressure sensor cannot be sufficiently limited.
Furthermore, in the system of Japanese Unexamined Patent Publication No. H06-201496, which determines that the atmospheric pressure sensor is abnormal when the rate of change in the measured atmospheric pressure value of the atmospheric pressure sensor becomes equal to or greater than the abnormality determination threshold value, the normal atmospheric pressure sensor could be erroneously determined as the abnormal atmospheric pressure sensor when the measured atmospheric pressure value of the atmospheric pressure sensor temporarily and rapidly drops at the time of application of water to the case.
The present invention is made in view of the above disadvantages, and therefore it is an objective of the present invention to provide an atmospheric pressure sensing apparatus that addresses at least one of the above disadvantages.
To achieve the objective of the present invention, there is provided an atmospheric pressure sensing apparatus for a vehicle. The atmospheric pressure sensing apparatus includes a case, an atmospheric pressure sensor and a measured atmospheric pressure value limiting means. The case receives a control circuit of an internal combustion engine of the vehicle. The atmospheric pressure sensor measures an atmospheric pressure and is received in the case. The measured atmospheric pressure value limiting means is for limiting a rapid change in a measured atmospheric pressure value, which is measured with the atmospheric pressure sensor.